A scanning probe microscope (SPM) is an apparatus for performing three-dimensional mapping of an interaction (atomic force, contact force, etc.) between a probe and a sample surface by scanning the probe or the sample in the XY or XYZ directions, while detecting the interaction between the probe and the sample surface, when they are in close proximity or contact to each other. SPM is the general term for a scanning tunneling microscope (STM), an atomic force microscope (AFM), a magnetic force microscope (MFM), and a scanning near-field optical microscope (SNOM). In particular, the AFM is most widely used of all the SPMs as an apparatus for obtaining information on configuration on a sample surface.
The AFM comprises a cantilever including a lever portion, a free end of which has a sharp projection (a probe having a sharp point), and the other end of which is fixed. The cantilever is brought into close proximity to a sample, such that the tip of the probe faces the surface of the sample. While the sample or the cantilever is scanned in the XY directions, the amount of displacement of the probe and the amount of elastic deformation (flexibility) of the lever portion, which vary due to an interaction (atomic force, contact force, etc.) between atoms at the tip of the probe and on the surface of the sample, are electrically or optically detected and measured. Thus, information on the sample, e.g., configuration, is detected three-dimensionally by relatively changing the positional relationship between the probe of the cantilever and the sample.
A method for detecting an amount of elastic deformation (flexibility) of the lever portion of an SPM is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publications Nos. 5-340718 and 9-15250, the contents of which are incorporated herein by reference. The detection of the amount of flexibility of the lever portion, disclosed in these references, is performed by means of a known xe2x80x9cdisplacement detecting sensor of an optical lever systemxe2x80x9d. In connection with the detection of the amount of flexibility of the lever portion, xe2x80x9ca method for detecting defocus by means of a critical angle prismxe2x80x9d, xe2x80x9ca displacement detecting sensor utilizing an optical interferometerxe2x80x9d, etc. are also known.
The SPM measurement by means of the displacement detecting sensor is performed as follows. A probe is scanned over a measurement region of a sample in the XY directions. The amount of flexibility of the lever portion in the measurement region is detected by the displacement detecting sensor whenever necessary. The detected amount of flexibility is imaged at resolution of an atomic level as sample information, such as configuration or magnetic power of the surface of the sample, and displayed on a monitor.
Cantilevers used in such an SPM have been mainly produced by an applied semiconductor IC manufacturing process, since the process was proposed, as disclosed in Thomas R. Albrecht and Calvin F. Quate, xe2x80x9cAtomic Resolution Imaging of a Nonconductor by Atomic Force Microscopyxe2x80x9d, J. Appl. Phys. 62 (1987), page 2599. This is because the process allows a cantilever to be produced with high accuracy in the order of microns at low cost by using a batch process.
The cantilevers on the market now include the following two types: cantilevers made of silicon nitride; and cantilevers made of silicon. The mainstream of the silicon nitride cantilevers is described in Thomas R. Albrecht et al., xe2x80x9cMicrofabrication of Cantilever Styli for the Atomic Force Microscopexe2x80x9d, J. Vac. Sci. Technol. A8, 3386 (1990). A detailed method for fabricating this type of cantilever is disclosed in U.S. Pat. No. 5,399,232, the contents of which are incorporated herein by reference. The mainstream of the silicon cantilevers is described in O. Wolter et al., xe2x80x9cMicromachined Silicon Sensors for Scanning Force Microscopyxe2x80x9d, J. Vac. Sci. Technol. B9, 1353 (1991). A detailed method for fabricating this type of cantilever is disclosed in U.S. Pat. No. 5,051,379, the contents of which are incorporated herein by reference.
[Problem 1A]
To fabricate a silicon nitride cantilever, first, a lever base material of silicon nitride and a supporting portion to be attached to an apparatus are produced separately, and then the two parts are adhered by anode adhesion or the like. The portion of the lever base material, which is not adhered to the supporting portion, functions as a lever portion.
Therefore, the length of the lever portion varies depending on accuracy of the adhesion between the lever base material and the supporting portion. The anode adhesion causes variance of about 10-30 xcexcm in length of the portion where the lever base material adheres to the supporting portion. Accordingly, variance in length of the lever portion is about 10-30 xcexcm in the silicon nitride cantilever.
To fabricate a silicon cantilever, a lever portion is formed by dry etching from one side of a silicon wafer, and a supporting portion made of silicon is formed by wet anisotropic etching from the other side on which the lever portion is not formed.
The length of the lever portion depends on the thickness of the silicon wafer, and displacement of a lever portion forming mask and a supporting portion forming mask formed on both sides of the silicon wafer. In general, variance in thickness of silicon wafers available on the market is at least 10 xcexcm. Further, the displacement of the aforementioned masks formed on both sides of a silicon wafer is about 10 to 20 xcexcm. Consequently, variance in length of the lever portion is about 10 to 30 xcexcm in the silicon cantilever.
Moreover, in the silicon cantilever, variance in thickness of each of the silicon wafers is directly reflected in the thickness of the lever portion. Therefore, in the silicon cantilever, variance in thickness of the lever portion is about 1 xcexcm.
In the cantilevers used in the SPM measurement, it is necessary that the spring constant of the lever portion be known. Further, in the cantilevers used in the SPM measurement in an oscillation mode for oscillating the cantilever, it is necessary that the resonance frequency, as well as the spring constant, of the lever portion be known. To obtain an accurate result of SPM measurement, it is desirable that variance in both the spring constant and the resonance frequency be little.
The frequency resonance of the lever portion of the cantilever is inversely proportional to the square of the length of the lever portion and proportional to the thickness thereof. The spring constant is inversely proportional to the cube of the length and proportional to the thickness. Therefore, to obtain a cantilever of little variance in the spring constant and the resonance frequency, it is necessary that the length and thickness of the lever portion be deviated as little as possible from the design values. In other words, a technique for forming a lever portion with high accuracy, in respect of the length and thickness, is required.
[Problem 1B]
Recently, an SPM, wherein the sample is raster-scanned at high speed of more than one screen per second, has been widely used by trial for observing microscopic movement of, for example, a biological sample. Further, the SPM technique has been also put to trial for high-density recording. Under these situations, there is a demand for a probe device shaped like a cantilever having a high resonance frequency in order to increase the input and output speeds.
The cantilever used for the SPM measurement at high-speed scanning is required to have a resonance frequency in the order of MHz or higher to increase the scanning speed. Further, the spring constant is required to be 40-50 N/m or smaller to prevent breakage of the probe or the sample in a case of contact therebetween.
In the conventional cantilever generally used in the SPM measurement, a resonance frequency is about 300 kHz, a spring constant is about 20-50 N/m, and a length of the lever portion is 100-200 xcexcm. Therefore, such a cantilever is not suitable for the resent SPM measurement using high-speed scanning.
As described above, the frequency resonance of the lever portion of the cantilever is inversely proportional to the square of the length of the lever portion and proportional to the thickness thereof, and the spring constant is inversely proportional to the cube of the length and proportional to the thickness. Therefore, to obtain a lever portion with a small spring constant and a high resonance frequency, it is necessary that the lever portion be short and thin.
For example, to realize a resonance frequency of 1 MHz with the same spring constant as that of a conventional silicon cantilever, which has a lever portion having a length of 120 xcexcm and a thickness of 3 xcexcm, the length of the lever portion must be 40 xcexcm or less, and the thickness thereof must be 1 xcexcm or less.
However, according to the conventional fabrication method as described above, the length of the lever portion inevitably varies between about 10 and 30 xcexcm. Therefore, it is substantially impossible to stably fabricate a cantilever with a lever portion of a length of 40 xcexcm or less.
Further, according to the conventional method for fabricating a silicon cantilever, variance in thickness of each of silicon wafers is reflected in the thickness of the lever portion. Since each of the silicon wafers has variance in thickness of 1 xcexcm or more, it is substantially impossible to stably fabricate a cantilever with a lever portion of a thickness of 1 xcexcm or less.
[Problem 2]
In the SPM measurement, one of the factors of determining the resolution of the measurement result is the aspect ratio, i.e., the sharpness of the probe of the cantilever used in the measurement, in particular, the tip of the probe. In other words, the resolution of the SPM measurement is greatly influenced by the sharpness of the probe used in the measurement. Therefore, it is desirable that the probe of the cantilever have a sharp tip.
The SPM has been widely used in various fields. At the beginning, it was mainly used to measure the roughness of a comparatively flat sample. Recently, however, it is used to measure a sample which has a large difference in level or a narrow groove. It is also used to measure a vertical wall in connection with the measurement of such a sample. In the recent SPM measurement, it is desirable that the probe of the cantilever be long and thin and the tip of the probe be sharp.
To form a lever portion of the silicon nitride cantilever described above, a silicon nitride film is formed on a silicon wafer in which an inverted pyramidal hole is formed. After the silicon nitride film is patterned, the silicon wafer is removed, so that a lever portion having a probe is formed. Therefore, the probe has a pyramidal projection formed of the silicon nitride deposited on the surfaces of the inverted pyramidal hole.
The inverted pyramidal hole is formed by processing a (100) oriented silicon wafer by wet anisotropic etching, using a mask which has a square opening. The hole of the inverted pyramidal hole thus formed is defined by four (111) planes of silicon.
Since the (111) plane is inclined at an angle of 54.7xc2x0 with respect to the (100) plane and the lever portion is formed of the silicon nitride film deposited on the (100) plane, the four side faces of the probe are all inclined at the angle of 54.7xc2x0 with respect to the (100) plane. Therefore, the vertex angle of the pyramidal probe of the silicon nitride cantilever is about 70xc2x0.
The silicon cantilever described before has a probe having a vertex angle of about 45xc2x0. Further, the following method for fabricating a silicon cantilever is recently produced. A triangular step is formed in a (100) oriented silicon wafer, such that the perpendicular from the vertex extends along the  less than 110 greater than  direction. The side surface of the step is oxidized to form an oxide film wall perpendicular to the (100) plane. Thereafter, the wafer is processed by wet anisotropic etching, thereby forming a lever portion having a probe at its top end.
The lever portion is formed of a portion of the triangular step which is etched in parallel with the (100) plane. The probe is formed of a triangular pyramid remaining at a top end portion of the triangular step. The side faces of the triangular probe are defined by two sides of the top end portion of the triangular step and a (111) plane of silicon exposed by the wet anisotropic etching.
As described before, since the (111) plane is inclined at an angle of 54.7xc2x0 with respect to the (100) plane, and the surface of the lever portion is constituted by the (100) plane, the side face of the probe constituted by the (100) plane is inclined at the angle of 54.7xc2x0 with respect to the lever surface. Therefore, the vertex angle of the pyramidal probe of the silicon cantilever is about 35xc2x0.
As can be understood from the above description, both in the silicon nitride cantilever and the silicon cantilever, the surface of the lever portion is formed of the (100) plane of silicon and at least one of the side faces of the probe is formed of the (111) plane of silicon. Therefore, according to the conventional methods, it is impossible to form a cantilever having a probe, all side faces of which are inclined at an angle greater than 54.7xc2x0 with the surface of the lever portion.
[Problem 3]
In the cantilevers produced by applying the process of manufacturing semiconductor ICs, the supporting portion supporting the lever portion has an attachment surface parallel to the surface of the lever portion. The cantilever is fixed to the SPM apparatus by attaching the attachment surface to the apparatus and pressed thereon by a fixing device, such as a spring.
The fixing device for pressing the supporting portion of the cantilever is arranged in a lower portion of the SPM apparatus in accordance with the shape of the cantilever. To avoid contact between the fixing device and a sample to be measured, the cantilever is attached so that the surface of the lever portion is inclined with respect to the surface of the sample.
However, the space obtained by attaching the cantilever obliquely is little, and contact between the fixing device and the sample cannot be completely avoided. Further, as the cantilever has become compact, the space obtained by attaching the cantilever obliquely becomes less. Therefore, it has been more difficult to avoid contact between the fixing device and the sample.
According to an aspect, the present invention was made to solve the aforementioned problems 1A and 1B. An object of the present invention is to provide a method for stably fabricating a cantilever having little variance in length and thickness of the lever portion. Another object of the present invention is to provide a method for constantly fabricating a cantilever including a lever portion which has a resonance frequency in the order of MHz and a spring constant of 40-50 N/m or smaller.
According to another aspect, the present invention was made to solve the aforementioned problem 2. An object of the present invention is to provide a cantilever having a sharp probe. Another object of the present invention is to provide a cantilever having a long probe.
According to still another aspect, the present invention was made to solve the aforementioned problem 3. An object of the present invention is to provide a cantilever and a mechanism for holding the same, which easily prevents contact between a supporting portion of the cantilever and a sample to be measured.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.